1. Field of the Invention
Methods consistent with the present invention relate to writing a reference servo signal for a hard disk drive, and more particularly, to writing a spiral reference servo signal which can compensate for an error due to a thermal expansion of a disk, and to compensating for the thermal expansion of a disk due to a temperature change.
2. Description of the Related Art
In general, a hard disk drive that is one of data storage devices reproduces data written on a disk or writes user data on the disk using a magnetic head, and thus, contributes to a computer system operation. As the hard disk drive becomes compact with high capacity and high density, a bit per inch (BPI) that is, a density in a disk rotational direction and a track per inch (TPI) that is, a density in a disk radial direction thereof increase, so that a more accurate mechanism is needed.
The hard disk drive consists of a bead disk assembly (HDA) and a printed circuit board (PCB) assembly to control the HDA. The HDA includes a head for storing and restoring information, a disk where information is written, a spindle motor for rotating the disk, an actuator arm and a voice coil motor (VCM) for moving the head, an outer disk crash stop (ODCS) and an inner disk crash stop (IDCS) for limiting a range of the actuator arm, etc.
The ODCS and the IDCS are bumper units for limiting a moving range of the actuator arm so as to prevent the head from moving to a position on the disk where servo information is not written.
To control the position of the head, servo information (position information) is written for each track. As the writing density of the hard disk drive increases, the number of tracks increases, thereby a portion of a time needed for writing the servo information on the disk gradually increases relative to the entire process.
A conventional servo writing method for writing servo information on the disk of the hard disk drive uses a highly accurate encoding system and a mechanical push pin. In this system, one end of the mechanical push pin is attached to a master actuator arm and the other end thereof is extended inside the hard disk drive through a servo write slot. The master actuator arm is controlled by a positioner of a high precision. In addition, a clock head is controlled to write a clock track that contributes as a timing reference during a servo writing process, on the disk.
In the above process, a non-repeatable run-out (NRRO), a disk flutter, a motor rocking, etc. may deteriorate the accuracy in position controlling. Furthermore, the use of the positioner and the encoder greatly increases costs related to the servo writing process, so that efficiency in production of the hard disk drive is deteriorated.
To overcome the above problems, an off-line servo writing method and a self-servo writing method have been developed.
In the off-line servo writing method, servo information is written to one or more disks using a servo track writing apparatus before the disks are installed on a hard disk drive. The method can improve accuracy compared to the conventional servo writing method.
In the self-servo writing method, final servo information is written to the disk based on the previously written reference servo information. According to this method, the quality of the final servo information is determined by the accuracy of the reference servo information. Also, since this method is hardly dependent on a servo writing apparatus, process cost is reduced.
For writing the reference servo information in the self-servo writing method, there are a three-burst method and a spiral method. In the three-burst method, three kinds of burst signals (reference servo signals) are written in a width wider than a final track width and the final servo signal is written according to the reference servo signal. In the spiral method, the reference servo signals are written in a spiral shape and the final servo signal is written based on the spiral reference servo signals. U.S. Pat. No. 5,668,679 (issued on Sep. 16, 1997) discloses the above spiral method.
FIG. 1 shows a conventional method for writing a spiral reference servo signal, which is disclosed in U.S. Pat. No. 5,668,679. Referring to FIG. 1, a disk 11 is installed on a rotatable spindle motor (not shown). A read/write head 12 is attached to an actuator arm 13 capable of adjusting a position of the read/write head 12. Reference numerals 17 and 18 denote two crash stops and reference numeral 14 denotes a voice coil motor. When the voice coil motor 14 is actuated so that the actuator 13 is moved with respect to the disk 11, the head 12 is positioned to an arbitrary position between positions R1 and R2 on the disk 11. R1 and R2 denote reference tracks located at arbitrarily different positions on the disk 11. If the head 12 moves across the disk 11 between the reference tracks R1 and R2 on the disk 11 at a constant speed and simultaneously writes a signal on the disk 11, the spiral reference servo signal 100 is written in a spiral shape as indicated in FIG. 1.
FIG. 2 shows the spiral reference servo signal written by the method shown in FIG. 1. The spiral reference servo signal is written at least as many times as the number of sectors, typically, twice the number of sectors. Reference tracks R1 and R2 indicate an outer circumferential limit and an inner circumferential limit on the disk 11. A clock signal 202 indicates an interval for writing the spiral reference servo signal. A trajectory in which the spiral reference servo signal is written is referred to as a spiral track 204. The spiral reference servo signal is a signal in which a plurality of bits are arranged as shown in the left side of FIG. 2 and has sync bits 206 arranged at a predetermined interval.
A process of writing the final servo signal with reference to the spiral reference servo signal is referred to as a servo copy process. In the servo copy process, final servo signals are written on tracks having a concentric shape formed with respect to the sync bits 206. Thus, the accuracy of the bit signals 206 must be strictly managed in writing the spiral reference servo signal. However, in writing the spiral reference servo signal, as the temperature of the disk 11 increases, a difference in temperature between an initial writing state and a final writing state exists accordingly.
Since the disk 11 is expanded by heat increase, the length of a spiral track gradually increases further as a writing time passes. When the head 12 moves in a radial direction at a constant speed from the initial writing state to the final writing state without considering the thermal expansion of the disk 11, sync between the spiral tracks that is, sync between the bit signals (206), is distorted.
FIG. 3 shows a state of the reference servo signal affected by the thermal expansion of the disk. In FIG. 3, an upper spiral track corresponds to a state in which the temperature is low, for example, in the initial writing state, while a middle spiral track corresponds to a state in which the temperature is high, for example, in the final writing state. When the temperature is high, the length of the spiral track increases as compared to a case in which the temperature is low, due to the expansion of the disk 11 in a radial direction thereof.
In both cases, when the writing speed of the spiral reference servo signal is identical, sync of the spiral reference servo signal written on the spiral track is maintained at the time of writing (C1=C2). However, as the temperature decreases, the expanded spiral track contracts as shown in a lower spiral track of FIG. 3, so that the sync of the spiral reference servo signal is distorted (C1≠C3).
The upper and lower spiral tracks of FIG. 3 correspond to an initially written spiral track and a finally written spiral track, respectively, on a reference disk to be used in a servo copy process. As a result, the servo copy process is performed based on the spiral reference servo signal having a broken sync, so that a correct final servo signal is hard to obtain.